Field of the Invention
The present invention relates to an operation method and a wind turbine configured to determine aerodynamic changes in the wind turbine blades due to ageing and degradation by environmental conditions, and to adapt the control method based on these changes.
Description of Related Art
It is known that rain, sand, dust, and other wind-born debris gradually chip and erode the critical leading edge of the fast moving wind turbine blades. Also, salt, pollution and sticky dust particles will accumulate on the outer surface adding weight and roughness to the wind turbine blade, thereby altering the aerodynamically shaped outer surfaces until rain or a maintenance wash cleans them off. Furthermore, ice can accumulate on the leading edges if the air contains droplets of super-cooled water, and lightning strikes may form cracks anywhere along the wind turbine blade despite that the wind turbine blade is outfitted with electrically conductive elements. Ageing affects the finish of the outer surface and the leading edge, thus altering their original shape and form. Any of these conditions may degrade the carefully designed aerodynamic profile of the wind turbine blade, therefore the wind turbine blade need to be cleaned and repaired to maintain the aerodynamic performance. Since the damage and aerodynamic degradation are primarily concentrated at the leading edges, special materials have to be applied to repair and protect the leading edges of the wind turbine blades, however servicing the blades is dangerous work as the tower is typically 90 meters high and subjected to wind gusts. Blade repair and other maintenance works are only occasionally performed as this work is expensive and adds to the downtime of the wind turbine.
The control systems of such wind turbines are typically tuned according to nominal aerodynamic specifications, where deviations from these nominal specifications cause the control system to de-tune and reduce the power production. This problem may be solved by tuning the control system according to partially degraded aerodynamic specifications in an effort to avoid control system misstep under heavy degradation, and to buffer the power production. However, the wind turbine is not operated optimally prior to the degradation and is not able to compensate for the progressive change of the aerodynamics occurring beyond these partially degraded specifications.
U.S. Pat. No. 8,405,239 B2 suggests using three different linear time domain models one for each operation range. A temporal variation table is generated for each operation range and compared to a current control parameter of the drive system. If the current control parameter deviates from the table value by more than 20%, then the control parameter is adjusted. It is stated that this allows the control system to compensate for changes in the dynamics of the drive system due to ageing. However, the teachings do not hint how the control system is re-tuned or how this control parameter should be adjusted. Furthermore, this solution does hint that the control system is able to detect the degradation of the blade aerodynamics.
U.S. Pat. No. 8,174,136 B2 discloses a maximum power point determination scheme based on an adaptive method for controlling the pitch and torque combined with a determination of a power coefficient. The power coefficient is determined over a time period as the ratio between the captured power and the available wind power. The pitch control signal or torque control signal is incrementally stepped by an increment value according to the changing power coefficient. The increment value is added to the current pitch control signal and, if the power coefficient is increased, the process is repeated. If the power coefficient is decreased, the increment value is negated and the process is repeated. In a similar manner, another increment value may be added to the current torque control signal.
It is stated that this solution allows the wind turbine to increase the power production at wind speeds below the rated wind speed. The averaged power is determined based on wind speeds measured by an anemometer. However, such an anemometer mounted on the wind turbine does not measure the actual wind speed, but the downstream wind speed after it has been significantly affected by the rotor. Large wind turbines have a rotor covering a great area, e.g., 7854 square meters [m2] for a rotor with rotor blades of 50 meters, thus the wind speed measured at a single anemometer position does not provide representative value of the wind speed impacting the entire rotor.
U.S. Patent Application Publication 2014/0241878 A1 discloses a method for monitoring the aerodynamic condition of the wind turbine blades. One or more sensors are arranged in example on the wind turbine blades or on the wind turbine tower for sensing one or more operating parameters of the wind turbine. A control unit analyses the received sensor data to determine the roughness state of the outer surface of the wind turbine blades. The control unit then adapts the pitch angle or rotor speed to compensate for the loss of the overall wind turbine performance. However, the teachings are silent about how the sensor data is analysed and how the pitch angle or rotor speed is adapted based on these sensor data.
U.S. Patent Application Publication 2015/0005966 A1 discloses a wind turbine control method, wherein a first scaling factor is calculated based on stored load conditions and a correction parameter is calculated based on measured operating conditions. The correction parameter is then used to calculate a second scaling factor, which is multiplied with the first scaling factor to determine a corrective action used to adjust the performance of the wind turbine. The purpose of this control method is to ensure that the design loads are not exceeded by monitoring the actual mechanical loading conditions and adapting the power output accordingly. This control method is not designed to monitor the extent of the aerodynamic degradation of the wind turbine blades, nor is it provides an effective control method for compensating for the changing aerodynamic properties of the wind turbine blades.
Therefore, there is a need for a method for monitoring the blade aerodynamic degradation, estimating when maintenance is needed, and adapting the wind turbine control according to the changing blade aerodynamics to maximize the power/energy production.